Enzymes that utilize rare transition metals are of interest to help us understand unique enzymic reactions. Formate dehydrogenase H (FDH) from E. coli is such an enzyme. FDH contains multiple redox centers, which include a molybdopterin cofactor, an iron-sulfur cluster and a natural selenocystein residue at its active site. This protein shares no sequence homology with other dehydrogenases which all have the well-known protein binding fold. We hope to understand the mechanism of this class of enzymes that utilizes molybdopterin and selenocystein to mediate their redox reaction. Another unique class of enzymes whose catalytic mechanism is not understood is that of the blood converting enzymes, a-N-acetylgalactosaminidase (a-NAGAL) and a-galactosidase (a-GAL). The ABO antigenic specificity in human blood is determined by the nature and linkage of monosaccharides at the end of the carbohydrate chains of either glycoproteins or glycolipids embedded in the cell membrane. An approach to providing large quantities of group O blood is to convert group A and B cells to group O by using the appropriate exoglycosidases. Understanding the mechanism of conversion of the carbohydrate chains through three-dimensional structure determination will hopefully provide the information necessary to produce an enzyme with both A and B converting function.